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2.8.5Atoms, Elements and Compounds

Tests for Water — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 22 of 26 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Testing for Water

Two colour-change tests show whether water is present in a liquid or a solid.

Anhydrous copper(II) sulfate
  • Anhydrous copper(II) sulfate This white powder turns from white to blue when water is present, forming hydrated copper(II) sulfate.
Cobalt(II) chloride paper
  • Cobalt(II) chloride paper This paper turns from blue to pink in the presence of water.
Positive result
  • Positive result A colour change in either test shows that water is present in the sample.
Reversibility
  • Reversibility Heating hydrated copper(II) sulfate turns it back to white, so the test can be repeated with the same powder.
TestColour without waterColour with water
Anhydrous copper(II) sulfateWhiteBlue
Cobalt(II) chloride paperBluePink

Part 2 of 5Testing Pure Water

Pure water has fixed physical constants that can be measured to confirm its purity.

1
Pure water freezes at 0 °C and boils at 100 °C at standard atmospheric pressure.
2
A sample with a different melting or boiling point contains dissolved substances and is not pure.
3
Measuring the boiling point with a thermometer in the boiling liquid confirms the sample is pure water.
4
Using both the copper(II) sulfate test and a boiling point check gives strong evidence of pure water.

Part 3 of 5Using the Tests

Using the Tests
Solids and liquids
  • Solids and liquids The copper(II) sulfate test can be used on solids and liquids, provided the sample can contact the powder.
Anhydrous
  • Anhydrous The word anhydrous means the compound contains no water, which is needed for the test.
Hydrated
  • Hydrated Blue copper(II) sulfate crystals are described as hydrated, since they contain water of crystallisation.
Dry solvent
  • Dry solvent Solvents for reactions are checked with anhydrous copper(II) sulfate to ensure they are dry.

Part 4 of 5Worked Example

Question

  • A colourless liquid is added to anhydrous copper(II) sulfate, which turns blue.
  • Explain what this shows and state the test for purity that could be carried out next.

  1. The colour change from white to blue shows that water is present in the liquid.
  2. The liquid is therefore water or contains water.
  3. To test purity, measure the boiling point; pure water boils at exactly 100 °C at standard pressure.

Answer

  • The liquid contains water, since anhydrous copper(II) sulfate turns from white to blue.
  • Measuring a boiling point of exactly 100 °C would support purity.

Part 5 of 5Exam Checklist & Tips

2.8.5 – Key Points for the Exam

  • Anhydrous Describes a substance that contains no water.
  • Hydrated Describes a substance that contains water of crystallisation.
  • Water of crystallisation Water molecules fixed in the structure of a crystal.
  • Standard pressure Atmospheric pressure at sea level, used when stating boiling points.

Common Mistakes

  • Stating that cobalt(II) chloride paper turns from pink to blue; the change is blue to pink.
  • Saying anhydrous copper(II) sulfate turns green when water is added; it becomes blue.
  • Stating that the test shows the liquid is pure; the colour change only shows that water is present.
  • Forgetting that a boiling point test depends on standard atmospheric pressure.

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